A radiation detector 10 can include an intrinsic region 11, an anode 12, and a cathode 18 as shown in FIGS. 1 and 2. Radiation detectors can include other components not shown, such as a guard ring. The anode 12 can be disposed at one face of the radiation detector, called an “anode face”. The cathode 18 can be disposed at the opposite face of the radiation detector, called a “cathode face”. The term “face” may be used in referring to either the “anode face” or the “cathode face”.
Radiation, such as x-rays, impinging upon the radiation detector can be absorbed by the detector. Each x-ray photon that is absorbed can create charge carriers, comprised of electron—hole pairs. Due to a potential difference between the anode and the cathode, positive charge carriers (holes) can be drawn to the anode and negative charge carriers (electrons) can be drawn to the cathode. The number of charge carriers created is proportional to the energy of the x-ray photon. Different materials produce different x-ray photon energies, thus the number of charge carriers can be used to determine the type of material that produced the x-rays.
The intrinsic region 11 can be divided into a full-charge collection region 17 and a partial-charge collection region 16. The full-charge collection region 17 is the portion of the intrinsic region 11 that is substantially within the width or area of the anode 12, or between the anode 12 and the cathode 18. The partial-charge collection region 16 is the portion of the intrinsic region 11 that is substantially outside the width or area of the anode 12.
When x-ray photons impinge upon the partial-charge collection region 16, typically only some of the generated holes reach the anode. The remaining holes can be lost to a guard ring or to recombination. As a result of less than all of the holes produced by the x-rays reaching the anode, an incorrect indication can be given of the type of material that produced the x-ray. In contrast, if x-rays impinge upon the full-charge collection region 17, substantially all of the charge carriers can reach the anode, resulting in a more accurate signal for indication of the type of material that produced such x-rays. Signals with a size that accurately reflect the number of charge carriers generated within the detector contribute to the “full energy peak” and can be used to determine the type of material that produced the x-rays. Signals that are reduced because of the loss of charge carriers to recombination or to the guard ring contribute to “background” and interfere with the determination of the type of material that produced the x-rays.
In order to avoid x-rays impinging on the partial-charge collection region 16, a collimator 14 can be used. The purpose of the collimator is to block x-rays that would otherwise impinge on the partial-charge collection region 16. Collimators can be attached to the radiation detector by standoffs 13. The standoffs 13 can be attached to the radiation detector and to the collimator by an adhesive. The standoffs can have a height h of about 100 to 200 micrometers such that the standoffs hold the collimator about 100 to 200 micrometers away from the face.
The collimator can have an opening 15 to allow x-rays to impinge on the full-charge collection region 17. Because x-rays do not all come directionally perpendicular to the surface of the radiation detector, such as x-rays 21a or 21d, the width w1, dimension or area of the collimator opening 15 is typically less than the width w2, dimension or area of the anode in order to block such x-rays.
Having a width w1, dimension or area of the collimator opening 15 that is less than the width w2, dimension or area of the anode can result in some undesirable blocking of x-rays. For example, x-ray 21b would impinge on the full-charge collection region 17 if it were not blocked by the collimator 14. Such undesirable blocking of x-rays can result in an undesirable longer collection time required for determination of the x-ray signal strength and thus the type of material that produced the x-ray.
Alignment of the collimator 14 above the radiation detector, such that the opening 15 in the collimator is centered over the anode 12, is important for blocking x-rays that would otherwise impinge on the partial-charge collection region 16 and for avoidance of blocking x-rays that that would impinge upon the full-charge collection region 17. Such collimator alignment can be a difficult manufacturing challenge and can frequently result in misalignment. For example, in FIG. 3, a radiation detector 30 is shown in which the opening 15 in the collimator is not centered over the anode 12. As a result, x-ray 21d, which would be blocked if the collimator 14 were properly aligned, is not blocked and impinges upon the partial-charge collection region 16. Also, x-ray 21c, which is directed at the full-charge collection region 17, would not be blocked if the collimator were properly aligned, but is now blocked due to collimator misalignment. Thus misalignment of the collimator can result in the undesirable results of x-rays impinging on the partial-charge collection region 16 and or blocking of x-rays that would otherwise impinge on the full-charge collection region 17.
As a collimator blocks an x-ray, it can absorb the energy of that x-ray and emit an x-ray that has an energy that is characteristic of the collimator material. Collimator materials that have high atomic numbers are more effective at blocking high energy x-rays. A disadvantage of the use of high atomic number collimator materials, however, is that x-rays emitted from the high atomic number collimator material can be similar in energy to the x-rays being measured, that were emitted from a sample material. Because of this similarity in x-ray energy between x-rays emitted from the sample material and x-rays emitted from the collimator, x-rays emitted from the collimator can interfere with determination of the type or quantity of material that is being analyzed. In contrast, a collimator material with a low atomic number can result in emission of x-rays that are so low in energy that there is little effect on detector performance. A problem of low atomic number collimator materials is that they can be less effective at blocking high energy x-rays emitted from the sample.